The field of the invention is electronic components, and particularly, fixed electrical resistors of the carbon composition type and methods of manufacturing the same.
Carbon composition resistors have been manufactured and widely used for many years. As disclosed in U.S. Pat. No. 1,835,267, issued to Lynde Bradley in 1931, early carbon composition resistors were large and bulky by today's standards. Despite this, however, they found wide application over the alternative forms of wire wound resistors and thin film resistors because they were more rugged and less susceptible to forming an open circuit during use. Also, after suitable manufacturing techniques had been developed, the composition resistors proved to be less expensive than the alternatives and a single standard size could be used through a range of resistance values from a few ohms to many megohms.
The art has continuously advanced throughout the ensuing years. For example, responding to the demand for smaller resistors, structures and manufacturing techniques were developed, such as those disclosed in U.S. Pat. Nos. 2,261,916; 2,271,774 and 2,302,564 which issued in 1941 and 1942. With the advent of miniaturized vacuum tubes and the transistor in the following decades, the demand for more rugged and even smaller 1/10 and 1/4 watt carbon composition resistors arose with the result that structures such as those disclosed in U.S. Pat. No. 3,238,490 issued to Homer Thomson in March, 1966, were developed.
The continued commercial success of the carbon composition resistor throughout the years is attributable in large measure to its continued lower cost and, therefore, any proposed improvement in existing carbon composition resistor structures must allow a cost advantage over alternative forms to be commercially viable. As evidenced by the above cited patents, past improvements in the structure of the carbon composition resistor have often been accompanied by corresponding advances in their method of manufacture to enable this continued cost advantage.
The flammability of components used in electronic applications has been of increasing concern to the electronics industry in recent years. Recognizing the need for flame resistant components, a number of resistance structures have been proposed to replace conventional resistors. One such proposed approach for achieving a flame resistant resistor is to construct it solely from thermally inert materials using wire as the resistance element. Another approach is to coat an otherwise flammable resistor with a nonorganic protective coating. Although many of these proposed structures have indeed substantially reduced the flammability of the component, they are not entirely satisfactory. First, the cost of many presently available flame resistant resistors is prohibitive for many applications where carbon composition resistors are presently used. In addition, although such flame resistant resistors may not themselves ignite when overloaded, the heat generated by the overload may affect the circuit board to which they are mounted or adjacent components.
In U.S. Pat. No. 3,887,893 issued to Ivan Brandt and Theodore von Alten on June 3, 1975, a cermet fixed resistor is disclosed which includes a thermal fuse connected in circuit with the resistance material. When the temperature of the substrate upon which the thermal fuse is mounted reaches a preset level, the thermal fuse opens circuit and the overload current is interrupted. The resistor is thus open circuited before the ignition temperature of any of its constituents or surrounding components is reached.